For unstructured grids, enable point merging when requested
For vtkCutter, point merging is not an explicit option. It is enabled by setting a vtkPointLocator that merges points. However, if a vtkNonMergingPointLocator is set as the point locator, then points will not be merged. Pass along the inferred merging option to vtk3DLinearGridPlaneCutter when execution is deferred to it.